1. Field of the Invention
The present invention relates to a thin film semiconductor device in which a plurality of thin film semiconductors are formed on a substrate having an insulating surface, which can be applied to a liquid crystal display device and the like.
2. Description of Related Art
An active matrix liquid crystal display device is known as one of devices employing a number of thin film transistors using thin film semiconductors. The active matrix liquid crystal display device is also referred to as what is called an AMLCD (Active Matrix Liquid Crystal Display), and is classified into some types according to the materials of the thin film transistors or the structure of the transistors. Since the thin film transistor is also referred to as what is called a TFT (Thin Film Transistor), the AMLCD is also referred to as a TFT liquid crystal.
With respect to the materials, there are an amorphous TFT type using amorphous silicon as a main material, a high temperature polysilicon TFT type using polycrystal silicon formed through a high temperature process more than 1,000° C. as a main material, a low temperature polysilicon TFT type using polycrystal silicon formed through a low temperature process of 600° C. as a main material, and the like. With respect to the structure of transistors, there are a bottom-gate type in which a gate electrode is disposed at a lower side, a top-gate type in which a gate electrode is disposed at an upper side, and the like.
Although there are features in the respective materials or the structure of the transistors, in the case of the amorphous TFT, since the mobility is small and not larger than 1 cm2/Vsec, in order to form a channel forming region between a gate insulating film and an active silicon layer with proper coordination, the gate insulating film and the active silicon layer must be continuously formed, and in order to prevent hydrogen in the amorphous silicon from drawing off, the amorphous silicon can not be heated up to a high temperature over 300° C. after formation thereof. Further, since the amorphous silicon has photosensitivity, it is desired to be as thin as possible, not larger than 300 Å, so that the bottom gate type reverse stagger structure is used.
Since the amorphous TFT has large resistance at an OFF-state, an OFF-state leak current is small, so that it is most suitable for a switching transistor of a pixel. However, since the mobility thereof is small, it is impossible to form a driver circuit such as a shift register on a substrate, and an external IC using crystalline silicon is always required. Accordingly, the amorphous TFT has problems in miniaturization and lowering the cost.
With respect to the high temperature TFT, since a high temperature process over 1,000° C. can be carried out, steps similar to so-called crystalline silicon can be used. Thus, a very stable process can be conducted and also the mobility thereof is about 100 cm2/Vsec, a driver circuit can be formed on a substrate. However, as a substrate capable of being used at a high temperature of 1,000° C., a substrate other than an expensive substrate such as quartz can not be used, so that the enlargement of the substrate is difficult, and the application of the high temperature TFT is restricted to a view finder with a diagonal of at most two inches or the like.
The low temperature polysilicon can be formed with the merits of both the amorphous TFT and the high temperature polysilicon TFT, and it has superior properties, that is, a TFT having a high mobility can be formed on a low cost normal glass substrate. Accordingly, it is also possible to form a driver circuit on the substrate and at the same time, to form a switching transistor for a liquid crystal pixel.
However, when the low temperature polysilicon is formed, in a step of crystallizing the amorphous silicon formed on the glass substrate, it is known by experiment that when the amorphous silicon film is thin, it can not be crystallized by the formation through low temperature thermal anneal of not larger than 600° C. Particularly, when the film thickness is not larger than 300 Å, the film can be hardly crystallized. Also in the case of the low temperature polysilicon, it becomes difficult to lower an OFF-state leak current at an OFF-state if the film thickness of silicon is not made thin, so that the thickness is desired to be as thin as possible. Although the silicon film with a thickness of not less than 300 Å can be used as a driver circuit or the like by using an LDD structure, it is preferable that the thickness is not larger than 300 Å in order to use the silicon film as a switching transistor for a pixel. In the case where the amorphous silicon with a thickness of not larger than 300 Å is crystallized, laser crystallization using a laser of a wavelength of not larger than 400 nm, such as an excimer laser, is effective. Crystallization is possible even for a film thickness of not larger than 300 Å by crystallization using a laser, and the laser crystallization is considerably used as a method of manufacturing a low temperature polysilicon TFT.
However, although crystallization by a laser is industrially possible when the substrate is small, when the substrate becomes large, crystallization of the entire surface of the substrate by a laser takes an extremely long time so that laser crystallization is not appropriate from the industrial standpoint. Further, since there is no laser device capable of annealing a large substrate at once, the entire of the substrate is crystallized by repeating partial laser crystallization. Accordingly, irregularity of laser irradiation directly causes irregularity of characteristics of TFTs.